1. Field of the Invention
The present invention generally relates to a system for feeding electric power to a terminal using a subscriber line, and in particular to a system having a power feeding circuit provided between exchange or switch equipment and the terminal.
2. Description of the Related Art
According to recommendations defined by ITU-T (ITU-Telecommunication Standardization Sector), ISDN exchange or switch equipment is equipped with an optional means for feeding power to network termination equipment through a subscriber line. Therefore, there are cases where the ISDN exchange equipment is not equipped with the power feeding means. To supply power from the side of the ISDN exchange equipment without a power feeding facility to the network termination equipment, there has been proposed a system having a power feeding adapter connected between the ISDN exchange equipment and the network termination equipment in Japanese Patent Unexamined Publication No. 8-251819.
More specifically, the ISDN exchange equipment is provided with a plurality of line termination circuits and the network termination equipment is provided with two network termination FQ5-331 2 circuits each corresponding to a different line termination circuit. The power feeding adapter is provided with first and second transformers which connect the two network termination circuits of the network termination equipment and the corresponding line termination circuits of the ISDN exchange equipment through a pair of 2-wire subscriber lines, respectively.
The power feeding adapter is further provided with a DC power source where the one output terminal of the DC power source is connected to the center tap of the secondary winding of the first transformer and the other output terminal of the DC power source is connected to the center tap of the secondary winding of the second transformer. The DC power is transferred to the network termination equipment through the pair of two 2-wire subscriber lines.
According to the above conventional system, however, a problem comes up in association with a subscriber line test which is an important function of the line termination circuit or subscriber line interface circuit. The subscriber line test is performed to measure various physical quantities including line resistance and capacitance between subscriber lines in a state where the subscriber line interface circuits are directly connected to the subscriber lines.
Therefore, in the case of the above conventional system, it is necessary to isolate the power feeding adapter from the subscriber lines when the subscriber line test is started and to connect it to the subscriber lines again when the subscriber line test has been completed. To achieve such a switching operation, the power feeding adapter needs to receive test starting and test completion signals from the exchange or switch equipment. However, such test starting and test completion signals are not provided in the case of the exchange equipment without a power feeding facility. In other words, to perform the subscriber line test, each subscriber line interface circuit needs to be provided with an additional circuit for generating the test starting and test completion signals, resulting in increased amount of hardware and complicated circuit configuration.